[Joe Ross]

I suspect that your digital sidecar isn't properly powered. It gets some power through the DB-37 cable, but not enough to send a full speed PWM to 4 motors. Assuming that your right motors are inverted in code, a left turn is minimum PWM to all motors and a right turn is full PWM to all motors.

Disconnect the DB-37 cable from the digital sidecar and make sure that all 3 power LEDs are brightly lit.